Among the conventionally-known gas combustion apparatus are gas engines capable of using a plurality of types of gas fuels, such as propane gas and butane gas, differing from each other in working pressure. One example of such gas engines is disclosed in Japanese Patent Application Laid-Open Publication No. 2008-106647 (hereinafter referred to as “the patent literature”). According to the disclosure of the patent literature, the gas fuels, such as propane gas and butane gas, differing from each other in working pressure are contained in respective gas containers, and these gas containers are provided in fluid communication with the gas engine via respective gas fuel supply passages. The gas fuel to be supplied to the gas engine is switchable between the propane gas and the butane gas via switching valves provided in the individual gas fuel supply passages. The gas fuel supply passages of the propane gas and butane gas (i.e., propane and butane gas fuel supply passages) join together (i.e., communicate with each other) at a position upstream of the gas engine.
Namely, by a human operator performing operation for switching operating states of the switching valves provided in the gas fuel supply passages, a desired gas fuel, selected from between the propane gas and the butane gas, can be supplied to the gas engine. Namely, the propane gas and the butane gas of different working pressures can be used switchably in the gas engine by the human operator selecting any desired one of the propane and butane gas to cause the selected gas to be supplied to the gas engine.
Sometimes, the gas engine is first driven with one of the gas fuels which has a high working pressure, i.e. the butane gas, and then driven with the other gas fuel which has a lower working pressure, i.e. the propane gas. In order to supply the propane gas to the gas engine, the switching valve of the propane gas fuel passage is opened.
Further, because the butane gas fuel supply passage and the propane gas fuel supply passage join together (i.e., communicate with each other) at a position upstream of the gas engine, the butane gas would stay in the propane gas fuel supply passage after the gas engine is driven with the butane gas. The butane gas staying in the propane gas fuel supply passage has a higher gas pressure than the propane gas. Therefore, if the switching valve of the propane gas fuel supply passage is opened to supply the propane gas to the gas engine, the residual butane gas staying in the propane gas fuel supply passage may undesirably flow to the propane gas container through the opened switching valve of the propane gas fuel supply passage.